1. Field of the Invention
The present invention relates generally to a lithium secondary battery provided with a positive electrode in which an aluminum foil containing manganese is used as a current collector, a negative electrode, and a non-aqueous electrolyte solution formed by dissolving lithium salt in a non-aqueous solvent, and more particularly, to the lithium secondary battery having excellent storage characteristics in a charged state through betterment of the non-aqueous electrolyte solution.
2. Description of the Related Art
Various efforts have been made to develop lithium secondary batteries widely used as a power source of portable electronic equipments such as portable telephones or digital cameras as quality of such portable electronic equipments has been improved.
For example, using an aluminum foil containing 0.6 to 2.0 wt % of manganese as a positive electrode current collector has been proposed in Japanese Patent Laid-Open No. Hei10(1998)-40921. According to the Patent Laid-Open, strength of the aluminum foil is improved by containing manganese. As a result, cracks in the positive electrode current collector by forth applied to the positive electrode which is caused by expansion of a negative electrode active material during charging is prevented, thus storage characteristics in a charged state is improved.
However, the storage characteristics of the lithium secondary battery in the charged state is not fully improved only by using the above-mentioned aluminum foil containing 0.6 to 2.0 wt % of manganese as the positive electrode current collector. The inventors of the present invention notice that it is because manganese contained in the aluminum foil elutes into the non-aqueous electrolyte solution while the lithium secondary battery is stored in the charged state, thus, the strength of the positive electrode current collector is decreased.
An object of the present invention is to attain a lithium secondary battery having excellent storage characteristics in a charged state.
Another object of the present invention is to prevent manganese contained in an aluminum foil from eluting into a non-aqueous electrolyte solution while a lithium secondary battery is stored in a charged state, thus to prevent strength of the aluminum foil from decreasing.
A lithium secondary battery according to the present invention is a lithium secondary battery provided with a positive electrode in which the aluminum foil containing manganese is used as a current collector, a negative electrode, and a non-aqueous electrolyte solution formed by dissolving lithium salt in a non-aqueous solvent wherein divalent manganese salt is added to said non-aqueous electrolyte solution.
Manganese contained in the aluminum foil is prevented from eluting into the non-aqueous electrolyte solution while the lithium secondary battery is stored in the charged state, thus the strength of the current collector is prevented from decreasing, as the result, the lithium secondary battery having excellent storage characteristics in the charged state is attained by adding the divalent manganese salt to the non-aqueous electrolyte solution.
In the current collector which is the above-mentioned aluminum foil containing manganese, when content by amount of manganese contained in the aluminum foil is less than 0.6 wt %, the strength of the current collector is not fully improved. On the other hand, when the content by amount of manganese is more than 2.0 wt %, the current collector gets too hard to process. Therefore, the aluminum foil containing 0.6 to 2.0 wt % of manganese is preferable as the above-mentioned current collector.
The strength of the current collector is improved by using such current collector. As the result, cracks in the positive electrode current collector by forth applied to the positive electrode which is caused by expansion of a negative electrode active material during charging is prevented, thus the storage characteristics in the charged state is improved.
Examples of material for such current collector include Al alloy 3003, 3203, 3004, 3104, and 3005 of Japanese Industrial Standards.
Lithium salt used in the above-mentioned non-aqueous electrolyte solution may be a sole type or a combination of 2 or more types of known lithium salt which has been conventionally generally used. As lithium salt, the lithium salt represented by a constitutional formula LiN(R1SO2)(R2SO2) wherein the R1 and R2 are of perfluoro alkyl group, the R1 and R2 may be the same or different from each other, and a total of number of carbon in the R1 and R2 is not less than 3 and the lithium salt represented by the constitutional formula LiC(R3SO2)(R4SO2)(R5SO2) wherein the R3, R4, and R5 are of perfluoro alkyl group, the R3, R4, and R5 may be the same or different from each other, and the total of the number of carbon in the R3, R4, and R5 is not less than 3 which are excellent in stability are preferably used.
In each of the above-mentioned lithium salt, when the lithium salt in which the total of the number of carbon in the above-mentioned R1 and R2 is more than 6 or the lithium salt in which the total of the number of carbon in the above-mentioned R3, R4, and R5 is more than 6 is used, viscosity of the non-aqueous electrolyte solution is increased, thus battery characteristics is decreased. Therefore, the lithium salt in which the total of the number of carbon in the above-mentioned R1 and R2 is in the range of 3 to 6 or the lithium salt in which the total of the number of carbon in the above-mentioned R3, R4, and R5 is in the range of 3 to 6 are preferably used. As such lithium salt, at least one type of lithium salt selected from the group consisting of LiN(C2F5SO2)2, LiN(CF3SO2)(C4F9SO2), and LiC(CF3SO2)3 are preferably used, and more preferably, LiN(C2F5SO2)2.
In preparing the non-aqueous electrolyte solution by dissolving the above-mentioned lithium salt in the non-aqueous solvent, concentration of lithium salt is preferably set in the range of 0.6 to 1.5 mol/l and more preferably, in the range of 0.9 to 1.3 mol/l.
As the non-aqueous solvent to dissolve the above-mentioned lithium salt, known non-aqueous solvents which have been conventionally generally used may be used. Examples of the such non-aqueous solvents include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethoxyethane, diethoxyethane, tetrahydrofuran, xcex3-butyrolactone, and dioxolane. The non-aqueous solvent may be a sole type or a combination of 2 or more types.
As the divalent manganese salt to be added to the non-aqueous electrolyte solution formed by dissolving lithium salt in the non-aqueous solvent, manganese (II) acetate Mn(CH3COO)2, manganese (II) sulfate MnSO4, manganese (II) benzoate Mn(C6H5COO)2, manganese (II) carbonate MnCO3, and manganese (II) nitrate Mn(NO3)2 and the like may be used. The divalent manganese salt may be a sole type or a combination of 2 or more types.
When the amount of the divalent manganese salt to be added to the non-aqueous electrolyte solution is less than 0.05 mol/l manganese contained in the aluminum foil is not fully prevented from eluting. On the other hand, when the amount of the divalent manganese salt to be added is more than 0.15 mol/l surplus Mn ion exercises a harmful influence. The result of both cases is that the storage characteristics is not fully improved. Therefore, the amount of divalent manganese salt to be added to the non-aqueous electrolyte solution is preferably set in the range of 0.05 to 0.15 mol/l.
The lithium secondary battery according to the present invention is characterized in that the aluminum foil containing manganese is used as the positive electrode current collector and that divalent manganese salt is added to the non-aqueous electrolyte solution. A positive electrode active material for use in its positive electrode and a negative electrode active material for use in its negative electrode are not particularly limited. Therefore, a known material that has been conventionally generally used may be used.
Examples of the positive electrode active material include lithium-containing transition metal oxides such as LiCoO2, LiNiO2, LiMn2O4, LiCo0.5Ni0.3Mn0.2O2 or LiMnO2, non-lithium-containing transition metal oxides such as MnO2, and sulfides such as TiS2.
Examples of the negative electrode active material include metal oxides such as TiO2 or Li2CuO2, carbon materials capable of intercalating and deintercalating lithium ions, lithium metal, and lithium alloys.
These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate specific embodiment of the invention.